U.S. patent application number 14/010697 was filed with the patent office on 2015-03-05 for determining non-contact state for a catheter.
This patent application is currently assigned to BIOSENSE WEBSTER (ISRAEL) LTD.. The applicant listed for this patent is BIOSENSE WEBSTER (ISRAEL) LTD.. Invention is credited to Natan Sharon Katz, Doron Moshe Ludwin, Erez Silberschein, Aharon Turgeman.
Application Number | 20150065851 14/010697 |
Document ID | / |
Family ID | 51392168 |
Filed Date | 2015-03-05 |
United States Patent
Application |
20150065851 |
Kind Code |
A1 |
Ludwin; Doron Moshe ; et
al. |
March 5, 2015 |
DETERMINING NON-CONTACT STATE FOR A CATHETER
Abstract
A method, including inserting a probe into a cavity in a
subject's body and receiving, from a force sensor in the probe,
first readings indicative of a first change in measured contact
forces between the probe and the cavity by less than a
predetermined limit over at least a predetermined interval of time.
The method continues by receiving second readings from the force
sensor when the second readings have changed by more than a
predetermined force threshold or location coordinates of the probe
have changed by at least a predetermined location change threshold.
The method continues by receiving third readings indicative of a
second change in the measured contact forces between the probe and
the cavity by less than the predetermined limit over at least the
predetermined interval of time. The method concludes by calibrating
a zero-force point for the force sensor according to the third
readings.
Inventors: |
Ludwin; Doron Moshe; (Haifa,
IL) ; Turgeman; Aharon; (Zichron Ya'acov, IL)
; Katz; Natan Sharon; (Kiryat Bialik, IL) ;
Silberschein; Erez; (Tel Aviv, IL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
BIOSENSE WEBSTER (ISRAEL) LTD. |
Yokneam |
|
IL |
|
|
Assignee: |
BIOSENSE WEBSTER (ISRAEL)
LTD.
Yokneam
IL
|
Family ID: |
51392168 |
Appl. No.: |
14/010697 |
Filed: |
August 27, 2013 |
Current U.S.
Class: |
600/409 ;
600/547; 600/587 |
Current CPC
Class: |
A61B 5/0538 20130101;
A61B 5/068 20130101; A61B 2034/2051 20160201; A61B 2560/0223
20130101; A61B 5/062 20130101; A61B 2090/065 20160201; A61B 5/1076
20130101; A61B 5/6885 20130101; A61B 2017/00725 20130101; A61B
18/1492 20130101; A61B 2505/05 20130101 |
Class at
Publication: |
600/409 ;
600/587; 600/547 |
International
Class: |
A61B 5/06 20060101
A61B005/06; A61B 18/14 20060101 A61B018/14; A61B 5/00 20060101
A61B005/00; A61B 5/107 20060101 A61B005/107; A61B 5/053 20060101
A61B005/053 |
Claims
1. A method, comprising: inserting a probe into a cavity in a body
of a subject; receiving, in a first period, from a force sensor in
the probe, first readings indicative of a first change in measured
contact forces between the probe and the cavity by less than a
predetermined limit over at least a predetermined interval of time;
receiving, in a second period after the first period, second
readings from the force sensor when one condition is met from a set
of conditions consisting of: the second readings having changed by
more than a predetermined force threshold in the second period; and
location coordinates of the probe having changed by at least a
predetermined location change threshold in the second period;
receiving from the force sensor, in a third period after the second
period, third readings indicative of a second change in the
measured contact forces between the probe and the cavity by less
than the predetermined limit over at least the predetermined
interval of time; and calibrating a zero-force point for the force
sensor according to the third readings.
2. The method according to claim 1, and comprising: receiving in
the first period first location signals indicative of a first
location change for the probe by more than a predetermined location
threshold; and receiving in the third period second location
signals indicative of a second location change for the probe by
more than the predetermined location threshold.
3. The method according to claim 1, wherein the measured contact
forces comprise magnitudes of the contact forces.
4. The method according to claim 1, wherein the measured contact
forces comprise directions of the contact forces.
5. The method according to claim 1, wherein the probe comprises a
first probe, the method further comprising: inserting a second
probe into the cavity in proximity to the first probe; receiving in
the first period first signals indicative of a first change in a
measured value of the proximity by less than a predetermined
proximity change threshold; and receiving in the third period
second signals indicative of a second change in the measured value
of the proximity by less than the predetermined proximity change
threshold.
6. The method according to claim 5, and comprising receiving the
first signals and the second signals from the force sensor.
7. The method according to claim 1, wherein the probe comprises a
magnetic field sensor, the method further comprising measuring the
location coordinates of the probe in response to magnetic fields
sensed by the sensor.
8. The method according to claim 1, wherein the probe comprises an
electrode, the method further comprising measuring the location
coordinates in response to an impedance of current flowing through
the electrode.
9. The method according to claim 1, wherein the predetermined limit
of the measured contact forces is equal to the predetermined force
threshold.
10. Apparatus, comprising: a probe configured to be inserted into a
cavity in a body of a subject; a force sensor located within the
probe; and a processor, configured; to receive from the force
sensor, in a first period, first readings indicative of a first
change in measured contact forces between the probe and the cavity
by less than a predetermined limit over at least a predetermined
interval of time, to receive, in a second period after the first
period, second readings from the force sensor when one condition is
met from a set of conditions consisting of: the second readings
having changed by more than a predetermined force threshold in the
second period, and location coordinates of the probe having changed
by at least a predetermined location change threshold in the second
period, to receive from the force sensor, in a third period after
the second period, third readings indicative of a second change in
the measured contact forces between the probe and the cavity by
less than the predetermined limit over at least the predetermined
interval of time, and to calibrate a zero-force point for the force
sensor according to the third readings.
11. The apparatus according to claim 10, and comprising a position
sensor located within the probe, and wherein the processor is
configured: to receive in the first period first location signals
from the position sensor indicative of a first location change for
the probe by more than a predetermined location threshold; and to
receive in the third period second location signals from the
position sensor indicative of a second location change for the
probe by more than the predetermined location threshold.
12. The apparatus according to claim 10, wherein the measured
contact forces comprise magnitudes of the contact forces.
13. The apparatus according to claim 10, wherein the measured
contact forces comprise directions of the contact forces.
14. The apparatus according to claim 10, wherein the probe
comprises a first probe, the apparatus further comprising a second
probe configured to be inserted into the cavity in proximity to the
first probe, and wherein the processor is configured: to receive in
the first period first signals indicative of a first change in a
measured value of the proximity by less than a predetermined
proximity change threshold; and to receive in the third period
second signals indicative of a second change in the measured value
of the proximity by less than the predetermined proximity change
threshold.
15. The apparatus according to claim 14, wherein the processor is
configured to receive the first signals and the second signals from
the force sensor.
16. The apparatus according to claim 10, wherein the probe
comprises a magnetic field sensor, and wherein the processor is
configured to measure the location coordinates in response to
magnetic fields sensed by the sensor.
17. The apparatus according to claim 10, wherein the probe
comprises an electrode, and wherein the processor is configured to
measure the location coordinates in response to an impedance of
current flowing through the electrode.
18. The apparatus according to claim 10, wherein the predetermined
limit of the measured contact forces is equal to the predetermined
force threshold.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is related to U.S. patent application
titled "Determining Absence of Contact for a Catheter," filed on
even date with the present application, and which is incorporated
herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates generally to contact
determination, and specifically to determination of absence of
contact of a catheter with body tissue.
BACKGROUND OF THE INVENTION
[0003] In an ablation procedure on target tissue, such as the
myocardium, verification of physical electrode contact with the
target tissue, as well as measurement of the force or pressure of
the contact, are important for controlling the delivery of ablation
energy to the tissue. Attempts in the art to verify electrode
contact with the tissue, and to accurately measure the contact
force, have been extensive, and various techniques have been
suggested. For example, U.S. Pat. No. 6,695,808, which is
incorporated herein by reference, describes apparatus for treating
a selected patient tissue or organ region. A probe has a contact
surface that may be urged against the region, thereby creating
contact pressure. A pressure transducer measures the contact
pressure. This arrangement is said to meet the needs of procedures
in which a medical instrument must be placed in firm but not
excessive contact with an anatomical surface, by providing
information to the user of the instrument that is indicative of the
existence and magnitude of the contact force.
[0004] As another example, U.S. Pat. No. 6,241,724, which is
incorporated herein by reference, describes methods for creating
lesions in body tissue using segmented electrode assemblies. In one
embodiment, an electrode assembly on a catheter carries pressure
transducers, which sense contact with tissue and convey signals to
a pressure contact module. The module identifies the electrode
elements that are associated with the pressure transducer signals
and directs an energy generator to convey RF energy to these
elements, and not to other elements that are in contact only with
blood.
[0005] A further example is presented in U.S. Pat. No. 6,915,149,
which is incorporated herein by reference. This patent describes a
method for mapping a heart using a catheter having a tip electrode
for measuring local electrical activity. In order to avoid
artifacts that may arise from poor tip contact with the tissue, the
contact pressure between the tip and the tissue is measured using a
pressure sensor to ensure stable contact.
[0006] U.S. Patent Application Publication 2007/0100332, which is
incorporated herein by reference, describes systems and methods for
assessing electrode-tissue contact for tissue ablation. An
electro-mechanical sensor within the catheter shaft generates
electrical signals corresponding to the amount of movement of the
electrode within a distal portion of the catheter shaft. An output
device receives the electrical signals for assessing a level of
contact between the electrode and a tissue.
[0007] U.S. Pat. No. 7,306,593, issued to Keidar et al., which is
incorporated herein by reference, describes a method for ablating
tissue in an organ by contacting a probe inside the body with the
tissue to be ablated, and measuring one or more local parameters at
the contact position using the probe prior to ablating the tissue.
A map of the organ is displayed, showing, based on the one or more
local parameters, a predicted extent of ablation of the tissue to
be achieved for a given dosage of energy applied at the position
using the probe. The given dosage of energy is applied to ablate
the tissue using the probe, and an actual extent of the ablation at
the position is measured using the probe subsequent to ablating the
tissue. The measured actual extent of the ablation is displayed on
the map for comparison with the predicted extent.
[0008] Impedance-based methods for assessing catheter-tissue
contact that are known in the art typically rely on measurement of
the magnitude of the impedance between an electrode on the catheter
and a body-surface electrode. When the magnitude is below some
threshold, the electrode is considered to be in contact with the
tissue. This sort of binary contact indication may be unreliable,
however, and is sensitive to changes in the impedance between the
body-surface electrode and the skin.
[0009] U.S. Patent Application Publication Nos. 2008/0288038 and
2008/0275465, both by Saurav et al., which are incorporated herein
by reference, describe an electrode catheter system having an
electrode adapted to apply electric energy. A measurement circuit
adapted to measure impedance may be implemented between the
electrode and ground as the electrode approaches a target tissue. A
processor or processing units may be implemented to determine a
contact condition for the target tissue based at least in part on
reactance of the impedance measured by the measurement circuit. In
another embodiment, the contact condition may be based on the phase
angle of the impedance.
[0010] Documents incorporated by reference in the present patent
application are to be considered an integral part of the
application except that to the extent any terms are defined in
these incorporated documents in a manner that conflicts with the
definitions made explicitly or implicitly in the present
specification, only the definitions in the present specification
should be considered.
SUMMARY OF THE INVENTION
[0011] An embodiment of the present invention provides a method,
including:
[0012] inserting a probe into a cavity in a body of a subject;
[0013] receiving, in a first period, from a force sensor in the
probe, first readings indicative of a first change in measured
contact forces between the probe and the cavity by less than a
predetermined limit over at least a predetermined interval of
time;
[0014] receiving, in a second period after the first period, second
readings from the force sensor when one condition is met from a set
of conditions consisting of:
[0015] the second readings having changed by more than a
predetermined force threshold in the second period; and
[0016] location coordinates of the probe having changed by at least
a predetermined location change threshold in the second period;
[0017] receiving from the force sensor, in a third period after the
second period, third readings indicative of a second change in the
measured contact forces between the probe and the cavity by less
than the predetermined limit over at least the predetermined
interval of time; and
[0018] calibrating a zero-force point for the force sensor
according to the third readings.
[0019] In a disclosed embodiment the method includes:
[0020] receiving in the first period first location signals
indicative of a first location change for the probe by more than a
predetermined location threshold; and
[0021] receiving in the third period second location signals
indicative of a second location change for the probe by more than
the predetermined location threshold.
[0022] The measured contact forces may include magnitudes of the
contact forces. Alternatively or additionally the measured contact
forces may include directions of contact forces.
[0023] In a further disclosed embodiment the probe consists of a
first probe, and the method further includes:
[0024] inserting a second probe into the cavity in proximity to the
first probe;
[0025] receiving in the first period first signals indicative of a
first change in a measured value of the proximity by less than a
predetermined proximity change threshold; and
[0026] receiving in the third period second signals indicative of a
second change in the measured value of the proximity by less than
the predetermined proximity change threshold.
[0027] The method may yet further include receiving the first
signals and the second signals from the force sensor.
[0028] In an alternative embodiment the probe includes a magnetic
field sensor, the method further including measuring the location
coordinates of the probe in response to magnetic fields sensed by
the sensor.
[0029] In a further alternative embodiment the probe includes an
electrode, the method further including measuring the location
coordinates in response to an impedance of current flowing through
the electrode.
[0030] Typically, the predetermined limit of the measured contact
forces is equal to the predetermined force threshold.
[0031] There is further provided, according to an embodiment of the
present invention, apparatus, including:
[0032] a probe configured to be inserted into a cavity in a body of
a subject;
[0033] a force sensor located within the probe; and
[0034] a processor, configured;
[0035] to receive from the force sensor, in a first period, first
readings indicative of a first change in measured contact forces
between the probe and the cavity by less than a predetermined limit
over at least a predetermined interval of time,
[0036] to receive, in a second period after the first period,
second readings from the force sensor when one condition is met
from a set of conditions consisting of:
[0037] the second readings having changed by more than a
predetermined force threshold in the second period, and
[0038] location coordinates of the probe having changed by at least
a predetermined location change threshold in the second period,
[0039] to receive from the force sensor, in a third period after
the second period, third readings indicative of a second change in
the measured contact forces between the probe and the cavity by
less than the predetermined limit over at least the predetermined
interval of time, and
[0040] to calibrate a zero-force point for the force sensor
according to the third readings.
[0041] The present disclosure will be more fully understood from
the following detailed description of the embodiments thereof,
taken together with the drawings, in which:
BRIEF DESCRIPTION OF THE DRAWINGS
[0042] FIG. 1 is a pictorial illustration of a probe system for
performing ablative procedures on a heart of a living subject,
according to an embodiment of the invention;
[0043] FIG. 2 is a first flowchart of steps followed by a processor
of the probe system, according to an embodiment of the
invention;
[0044] FIG. 3 is a second flowchart of steps followed by the
processor, according to an embodiment of the present invention;
and
[0045] FIG. 4 is a schematic graph of force magnitude vs. time for
the probe system, according to an embodiment of the present
invention.
DETAILED DESCRIPTION OF EMBODIMENTS OVERVIEW
[0046] In a medical ablation procedure, such as ablation of heart
tissue, it is extremely useful to be able to measure the force
applied (to the tissue) while the tissue is being ablated. This is
because the force applied is a key parameter governing the amount
of tissue ablated for a given ablation energy input to the tissue.
The ablation is typically provided by a probe comprising an
ablation electrode at its distal end. To measure the force a force
sensor may be incorporated into the distal end, but such force
sensors known in the art typically drift, i.e., even if the force
exerted on the sensor is constant, readings from the sensor change.
Such drift may be compensated for by zeroing the sensor
periodically, typically before applying ablation energy. However,
the zeroing of the sensor should only be applied if the sensor is
not contacting tissue, i.e., the sensor is in a state where the
force on it is effectively zero (such a state is typically achieved
if the sensor is surrounded by blood in the heart chamber, and is
not contacting a heart wall).
[0047] Embodiments of the present invention detect such a state for
the sensor, herein termed a zeroing state. The force sensor is
assumed to be in a zeroing state if over at least a predetermined
interval of time force readings from the sensor change by less than
a predetermined force limit. To ensure that the sensor is in the
zeroing state, the probe having the force sensor is typically also
assumed to change its location during the predetermined time
interval by more than a predetermined location threshold.
[0048] In addition to detecting a zeroing state for the sensor,
embodiments of the present invention auto-zero the sensor, i.e.,
they calibrate a zero-force point for the force sensor. In order to
auto-zero the sensor, received signals from the sensor are checked
to detect a situation wherein the sensor is in a first zeroing
state, then in a non-zeroing state (such as if the sensor indicates
it is touching tissue), and then in a second zeroing state. Once
such a situation is detected, force readings from the second
zeroing state may be used as calibration values that zero the
sensor.
[0049] In some embodiments the probe is in proximity to another,
second, probe. The inventors have found that changes in proximity
between the probes may reduce the accuracy of the calibration
values referred to above. In these embodiments the first probe may
be assumed to be in the zeroing state if, in addition to the force
condition described above, a measured value of the change in
proximity is less than a predetermined proximity change
threshold.
[0050] The inventors have found that, from measurements on actual
cases using embodiments of the present invention, there is an
extremely high probability of auto-zeroing the sensor when the
sensor does not contact tissue. In addition, there is an extremely
high probability of not auto-zeroing when the sensor does contact
tissue.
System Description
[0051] In the following description, like elements in the drawings
are identified by like numerals, and the like elements are
differentiated as necessary by appending a letter to the
identifying numeral.
[0052] Reference is now made to FIG. 1, which is a pictorial
illustration of a probe system 10 for performing ablative
procedures on a heart 12 of a living subject 13, according to an
embodiment of the invention. The system comprises a probe 14,
typically a catheter, which is percutaneously inserted by an
operator 16 through the patient's vascular system into a chamber or
vascular structure of the heart. The operator, who is typically a
physician, brings the probe's distal tip 18 into contact with a
heart wall 19 at an ablation target site. Optionally, electrical
activation maps may then be prepared, according to the methods
disclosed in U.S. Pat. Nos. 6,226,542, and 6,301,496, and in
commonly assigned U.S. Pat. No. 6,892,091, whose disclosures are
herein incorporated by reference. One commercial product embodying
elements of system 10 is available as the CARTO.RTM. 3 System,
available from Biosense Webster, Inc., 3333 Diamond Canyon Road,
Diamond Bar, Calif. 91765.
[0053] Areas determined to be abnormal, for example by evaluation
of the electrical activation maps, can be ablated by application of
thermal energy, e.g., by passage of radiofrequency electrical
current through wires in the probe to one or more electrodes at
distal tip 18, which apply the radiofrequency energy to the
myocardium. The energy is absorbed in the tissue, heating it to a
point (typically about 50.degree. C.) at which it permanently loses
its electrical excitability. This procedure creates non-conducting
lesions in the cardiac tissue, which disrupt the abnormal
electrical pathway causing the arrhythmia. The principles of the
invention can be applied to different heart chambers to treat many
different cardiac arrhythmias.
[0054] Probe 14 typically comprises a handle 20, having suitable
controls on the handle to enable operator 16 to steer, position and
orient a distal end 21 of the probe as desired for the ablation. To
aid the operator, the distal end of probe 14 contains a position
sensor 38 that provides signals to a positioning processor 22,
located in a console 24.
[0055] Ablation energy signals and other electrical signals can be
conveyed to and from heart 12 through an electrode 32 located at
distal tip 18 via a cable 34 to console 24. Electrode 32 may also
be referred to herein as the ablation electrode. There may be other
electrodes (not shown) located at the distal end that are used for
ablation. Pacing signals and other control signals may be conveyed
from the console through cable 34 and electrode 32, or via the
other electrodes at the distal end, to the heart; these signals may
be conveyed in parallel with any ablation energy signals, typically
by using frequency multiplexing for the different signals.
[0056] Factors affecting the ablation generated by the ablation
energy input to the tissue being ablated comprise, inter alia, the
contact force applied to the tissue during the ablation process. In
order to measure the contact force, the distal end of probe 14
comprises a force sensor 36. Force or pressure sensors that are
suitable for use in a probe are well known in the art. For example,
U.S. Patent Application Publications 2007/0100332 and 2009/0093806,
whose disclosures are incorporated herein by reference, describe
methods of sensing contact pressure between the distal tip of a
probe and tissue in a body cavity using a force or pressure sensor
embedded in the probe. However, force sensor 36 may comprise any
other force or pressure sensor known in the art.
[0057] Wire connections 35 link the console with body surface
electrodes 30 and other components of a positioning sub-system.
Electrode 32 and body surface electrodes 30 may be used to measure
tissue impedance at the ablation site as taught in U.S. Pat. No.
7,536,218, issued to Govari et al., which is herein incorporated by
reference. A temperature sensor (not shown), typically a
thermocouple or thermistor, may be mounted on or near electrode
32.
[0058] Positioning processor 22 is an element of a positioning
subsystem (of system 10) which measures location and orientation
coordinates of probe 14, using position sensor 38.
[0059] In one embodiment, the positioning subsystem comprises a
magnetic position tracking arrangement that determines the location
and orientation of probe 14 by generating magnetic fields in a
predefined working volume in the vicinity of the probe, using field
generating coils 28. These fields are sensed by position sensor 38
and the sensed fields are used to determine location and
orientation coordinates for the probe.
[0060] In console 24 an impedance module 40 is configured to
measure an impedance of current flowing between ablation electrode
32 and body electrodes 30. In some cases measurements of the
impedance may be used to estimate a location of electrode 32.
[0061] As noted above, probe 14 is coupled to console 24, which
enables operator 16 to observe and regulate the functions of the
probe. Console 24 includes a processor 25, preferably a computer
with appropriate signal processing circuits, which operates system
10. Processor 25 is coupled to drive a monitor 29. The signal
processing circuits typically receive, amplify, filter and digitize
signals from probe 14, including signals generated by the
above-noted sensors and a plurality of location sensing electrodes
(not shown) located distally in the probe. The digitized signals
are received and used by the console and the positioning subsystem
to compute the location and orientation of probe 14 and to analyze
the electrical signals from the electrodes.
[0062] Typically, during a medical procedure involving the
measurement of contact force by force sensor 36, the output of the
sensor drifts, even though the force on the sensor may be constant.
Sensor 36 typically measures the magnitude and direction of the
force, and the drift may be in one of these variables, or in both
of them. The drift is typically caused by changes in parameters of
physical elements associated with the sensor, such as gain changes
of amplifiers and/or dimensional changes of parts of the sensor.
The drift may be compensated for by zeroing the sensor, but the
zeroing should only be performed when there is no contact between
the distal tip of the probe and a solid object such as wall 19 of
heart 12. Embodiments of the present invention detect periods when
no such contact exists, and auto-zero sensor 36 during these
periods.
[0063] In some embodiments a second probe 50, generally similar to
probe 14, hereinbelow also termed the first probe, is located
within heart 12 so that a distal end 52 of the second probe is in
proximity to distal end 21 of the first probe. An electrode 54 is
located at a distal tip 56 of the second probe, and the electrode
is connected via a cable 58 to console 24. A position sensor 60,
generally similar to position sensor 38, is also located within
distal end 52 of the second probe.
[0064] In some cases, a change of the proximity of distal end 21 of
the first probe to distal end 52 of the second probe alters
readings of force sensor 36 in the first probe. The altered
readings typically reduce the accuracy of, or even completely
invalidate, any zero-force point derived from the readings during
calibration of the sensor.
[0065] In cases of the presence of second probe 50, embodiments of
the present invention use readings from force sensor 36 to estimate
the proximity of the two distal ends, and to quantify the proximity
numerically in a proximity index (PI) parameter. As is explained in
more detail below, force sensor 36 is only zeroed if a change in
the proximity index, .DELTA.PI is less than a preset value.
[0066] FIG. 2 is a first flowchart 100 of steps followed by
processor 25 and FIG. 3 is a second flowchart 200 of steps followed
by the processor, in auto-zeroing sensor 36, according to
embodiments of the present invention. Flowchart 100 is a
"high-level" flowchart of the steps followed by the processor in
determining when, and with what values, sensor 36 of the first
probe is to be auto-zeroed. Flowchart 100 assumes that sensor 36 is
in or is not in a "zeroing state." Flowchart 200 is a "low-level"
flowchart, providing the steps used by processor 25 in deciding
when sensor 36 is in a zeroing state.
[0067] Except where otherwise stated, the descriptions for the two
flowcharts assume that second probe 50 is also present in heart
12.
[0068] To perform the steps of the two flowcharts, processor 25
uses values of force, proximity index, and location that are
measured for distal tip 18. The force sensor is able to measure
both the magnitude and direction of the force, but for simplicity
in the following description, only the magnitude of the force is
considered. Those having ordinary skill in the art will be able to
adapt the description, mutatis mutandis, to account for changes in
the direction of the force, typically by calculating the magnitude
of the change of force vector.
[0069] For both flowcharts, the processor uses predetermined
threshold values for a force change, a proximity index change, a
location change, and a time period. The force, proximity index, and
location changes, and the time period threshold values are
respectively represented by the symbols .DELTA.F.sub.t,
.DELTA.PI.sub.t, .DELTA.L.sub.t, and .DELTA.T.sub.t. In a disclosed
embodiment .DELTA.F.sub.t=1 g, .DELTA.PI.sub.t=1.5,
.DELTA.L.sub.t=10 mm, and .DELTA.T.sub.t=1000 ms.
[0070] Processor 25 typically runs the two flowcharts in
parallel.
[0071] For high level flowchart 100, in a first zero identifying
step 102, processor 25 uses measurements of time, force, proximity
index and location to identify that force sensor 36 is in a zeroing
state. Flowchart 200 provides details of how the measurements are
used to determine of the sensor is in a zeroing state. At the
beginning of step 102, the processor saves initial values of time
T.sub.init, force F.sub.init, proximity index PI.sub.init, and
location L.sub.init. While in step 102, the processor continues to
acquire values of the time, the force, the proximity index, and the
location. A zeroing state for the force sensor corresponds to a
state where values of the time, force, proximity index and location
for the sensor indicate that the sensor distal tip of the probe is
not in contact with any solid surface, and that the proximity of
the two probe distal ends has not changed significantly. However,
in a zeroing state there is typically drift in the signal output by
the force sensor. As is shown in flowchart 200, the zeroing state
corresponds to, for at least predetermined time period
.DELTA.T.sub.t, the force registered by the force sensor changing
by less than predetermined force change threshold value
.DELTA.F.sub.t, the distal end moving by more than predetermined
distance .DELTA.L.sub.t, and the proximity index changing by less
than predetermined proximity index change threshold
.DELTA.I.sub.t.
[0072] In a termination step 104, the processor uses measurements
of the force and of the location to establish that the force sensor
is no longer in its zeroing state. The processor invokes
termination step 104 if either the force change measured by sensor
36 exceeds .DELTA.F.sub.t or if the location of distal tip 18
changes by more than .DELTA.L.sub.t. At this point the processor
saves current values of the force F.sub.current and calculates a
first zeroing step force change .DELTA.F.sub.1 according to
equation (1):
.DELTA.F.sub.1=|F.sub.current-F.sub.init| (1)
[0073] After completing termination step 104, in a change state
step 105 the probe enters a change state, where it is no longer in
a zeroing state.
[0074] In a second zero identifying step 106, processor 25
identifies that force sensor 36 is in a second zeroing state,
substantially repeating the procedure of step 102. While in the
second zeroing state, the processor calculates, on a continuing
basis, updated values of force changes, .DELTA.F.sub.2, using an
initial force saved on entering step 106 and an equation
corresponding to equation (1).
[0075] In a comparison step 108, the processor compares the changes
of values of the force of zeroing state step 106 with those saved
in step 104. I.e., the processor compares values of .DELTA.F.sub.1
and .DELTA.F.sub.2. Providing that the difference of the force
changes is within a predetermined limit, typically the
predetermined force change threshold .DELTA.F.sub.t, in an
auto-zero step 110 the processor is able to auto-zero the sensor,
typically by using the most recent values acquired from the force
sensor in step 106 as zero-point values for the sensor. In one
embodiment the zero-point values are an average of the most recent
signal values taken over a predetermined time period, such as
is.
[0076] If in comparison step 108 the difference of the force
changes is not within its predetermined limit, the flowchart
returns to step 102, and the processor reiterates the steps of the
flowchart.
[0077] Review of flowchart 100 shows that, for each iteration of
the flowchart, in steps 102 and 104 the processor generates and
saves a set of values {Z.sub.n}={T.sub.init, T.sub.current,
F.sub.init, F.sub.current, PI.sub.init, PI.sub.current, L.sub.init,
L.sub.current}.sub.n, where n is the number of the iteration. In
some embodiments, in step 108, the processor compares force change
values found in step 106 with all of the saved force change values
of previous iterations. If any one of the comparisons is valid,
then the comparison step 108 is assumed to return positive and the
processor uses the values in the second zeroing step to auto-zero
the force sensor.
[0078] FIG. 3 is flowchart 200, which comprises steps performed by
processor 25 in deciding if force sensor 36 is in a zeroing state.
In some cases, the force sensor may be in an intermediate state, as
explained below. In making its decisions, the processor uses
threshold values .DELTA.F.sub.t, .DELTA.PI.sub.t, .DELTA.L.sub.t,
and .DELTA.T.sub.t, referred to above.
[0079] For the steps of the flowchart, which mainly comprises
comparisons, processor 25 performs each of the steps on a
continuing basis so that the sequence of steps is performed
reiteratively as a loop. For each loop iteration there is a new
value of time, and typically new values of force, location or
proximity index. In some embodiments flowchart 200 is implemented
as a state machine. From the flowchart description herein, such a
state machine will be apparent to those having ordinary skill in
the art.
[0080] In an initial state step 202 the processor saves initial
values of time T.sub.init, force F.sub.init, proximity index
PI.sub.init, and location L.sub.init.
[0081] In a first comparison step 204, the processor measures a
current value of the force F.sub.current and a current value of the
proximity index PI.sub.current. Using the initial force and
proximity index values from step 202, the processor calculates
changes in values of force and proximity index according to
equations (2):
.DELTA. F = F current - F init .DELTA. PI = PI current - PI init }
( 2 ) ##EQU00001##
[0082] The processor then determines if the expression in
expression (3) returns true or false:
.DELTA.F<.DELTA.F.sub.t and .DELTA.PI<.DELTA.PI.sub.t (3)
[0083] The force comparison and the proximity index comparison are
preliminary tests which are typically true if the distal tip is not
in contact with wall 19 of the heart, and if the proximity index
has not changed appreciably.
[0084] If expression (3) returns true, then the flowchart continues
with further comparisons, starting with a second comparison 206:
.DELTA.T>.DELTA.T.sub.t. If expression (3) returns false, the
flowchart returns to initial step 202.
[0085] If second comparison 206 returns false, then force sensor 36
is in a first waiting state 212, waiting for the first comparison
to have been true for threshold time .DELTA.T.sub.t, and the
flowchart returns to first comparison 204.
[0086] If second comparison 206 returns true, the flowchart
continues to a third comparison 208, .DELTA.L>.DELTA.L.sub.t OR
.DELTA.F>.DELTA.F.sub.t. In comparison 208 the processor
respectively checks if the sensor has moved by more than the
threshold distance .DELTA.L.sub.t, or if the force has changed by
more than the threshold force .DELTA.F.sub.t.
[0087] If comparison 208 returns false, then force sensor 36 is in
a second waiting state 214 where the force sensor is waiting to
move more than the threshold distance .DELTA.L.sub.t or for the
force change to exceed the threshold force .DELTA.F.sub.t.
[0088] If comparison 208 returns true, the flowchart continues to a
fourth comparison 218, wherein the initial values F.sub.init and
proximity index PI.sub.init are updated to the respective values
when the processor enters the comparison.
[0089] In comparison 218 the processor checks that, over a time
period greater than the time threshold .DELTA.T.sub.t, the force
has changed by less than the force threshold .DELTA.F.sub.t and the
proximity index has changed by less than the proximity index
threshold .DELTA.PI.sub.t.
[0090] If comparison 218 returns true, the force sensor is in a
zeroing state 210.
[0091] If comparison 218 returns false, the force sensor is in a
third waiting state 216, where the sensor is waiting for comparison
218 to become true.
[0092] As is apparent from comparison 218, the force sensor is in
zeroing state 210 if, for at least a predetermined time period
.DELTA.T.sub.t, the force registered by the force sensor changes by
less than predetermined force threshold value .DELTA.F.sub.t and
the proximity index changes by less than predetermined proximity
index change threshold .DELTA.PI.sub.t. The condition of comparison
218 typically only holds if the distal tip does not contact wall 19
of the heart and if there has been no appreciable proximity index
change.
[0093] The description above for flowcharts 100 and 200 has assumed
the presence of probe 50, and that the distal ends of probes 14 and
50 may be in proximity to each other. Those having ordinary skill
in the art will be able to adapt the description, mutatis mutandis,
for the case when there is no probe distal end proximate to the
distal end of probe 14. Such adaptation may comprise, for example,
equating .DELTA.PI, in the conditions above regarding the proximity
index, to zero.
[0094] FIG. 4 is a schematic graph of force magnitude vs. time,
according to an embodiment of the present invention. The graph
illustrates values of time and the force magnitude, derived from a
vectorial force, as measured for distal tip 18. While the time and
the force are being measured, location and proximity values for the
distal tip are also measured, but for simplicity graphs of these
values vs. time are not shown. The force magnitude is measured in
grams (g), and the time is measured in seconds (s). The graph is
divided into three time periods, T.sub.1-T.sub.2, T.sub.2-T.sub.3,
and T.sub.3-T.sub.4, also respectively referred to herein as
periods A, B and C.
[0095] Considering period A, and applying values from the graph of
this period to flowchart 200, if all of the conditions of
expression (4) are assumed to be true, then at time T.sub.2 the
force sensor is in zeroing state 210.
F 2 - F 1 < .DELTA. F t T 2 - T 1 > .DELTA. T t .DELTA. PI A
< .DELTA. PI t .DELTA. L A > .DELTA. L t } ( 4 )
##EQU00002##
[0096] The first two comparisons in expression (4) are illustrated
in the graph; the third and fourth comparisons are from measured
proximity index and location changes, .DELTA.PI.sub.A and
.DELTA.L.sub.A, during period A.
[0097] Similarly for period C, if all of the conditions of
expression (5) are assumed to be true, then at time T.sub.4 the
force sensor is in zeroing state 210.
F 4 - F 3 < .DELTA. F t T 4 - T 3 > .DELTA. T t .DELTA. PI C
< .DELTA. PI t .DELTA. L C > .DELTA. L t } ( 5 )
##EQU00003##
[0098] The graph for periods A and C illustrate the case where
distal tip 18 is not in contact with wall 19.
[0099] Considering period B, if F.sub.3-F.sub.2>.DELTA.F.sub.t
then the force sensor is not in a zeroing state. The graph for
period B illustrates the case where distal tip 18 is in contact
with wall 19.
[0100] The situation illustrated by the graph, of a first and
second zeroing state separated by a period in time where there is a
change of force greater than the threshold force, may be applied to
flowchart 100. Steps 102, 104, 105, and 106 all apply. Assuming
comparison 108 is valid, then in step 110 the probe is auto-zeroed
at time T.sub.4 using values from time period C.
[0101] It will be appreciated that the embodiments described above
are cited by way of example, and that the present invention is not
limited to what has been particularly shown and described
hereinabove. Rather, the scope of the present invention includes
both combinations and subcombinations of the various features
described hereinabove, as well as variations and modifications
thereof which would occur to persons skilled in the art upon
reading the foregoing description and which are not disclosed in
the prior art.
* * * * *